Nitropyrenes - a group of chemicals present in mobile and stationary combustion emmissions are potent mutagens for Salmonella and cultured hamster cells. They do not appear to be mutagenic for human (normal or xeroderma pigmentosum) cells. It is proposed to investigate the basis of the action of nitropyrenes in these cells. The biotransformation, the formation of DNA-adducts, and the influence of enzyme modifiers thereunto will be investigated.